This invention relates generally to pulse combustion, and more particularly to the provision of a pulse combustion heat engine for temperature conditioning and pressurizing fluids. In an illustrated application, a pulse combustion heat engine is used to drive a reciprocating compressor in a reversible refrigeration system for heating and cooling fluids.
It is known to use internal combustion engines to drive free piston compressor devices as shown in U.S. Pat. Nos. 1,657,641 and 3,986,796. These arrangements are characterized by multiple valving functions and apparatus, since the piston provides both an engine function and a fluid compression function. The piston is returned after each compression stroke by the energy stored in the fluid of a "bounce" cylinder or a mechanical device such as a spring. Each of these patents also discloses the use of reservoirs of pressurized fluid as a starting fluid chamber to initially begin piston movement at start-up.
It is also known to use internal combustion engines to drive two independent compressors or compressed air sources as taught in U.S. Pat. No. 4,205,638. A power piston drives a pair of pistons mounted within angularly intersecting cylinders respectively using a working fluid and a mechanical connection. One of the pistons is used to provide a source of compressed air for injection into the combustion chamber of the engine.
In contrast with the foregoing prior art devices and techniques, applicants are not aware of the prior use of pulse combustion to drive a reciprocating or oscillating element for pressurizing a fluid, nor the use of a pulse combustion heat engine to power a compressor combined with the recovery of the engine-rejected heat. The use of the alternating pressures of the combustion gases in a pulse combustion system to both drive a reciprocating element and generate an oscillating flow field to enhance heat transfer provides efficiency improvements not previously achieved in the art.
In accordance with the invention, the pulse combustion heat engine is arranged to power a reversible refrigeration system or device and the engine-rejected heat is recovered in a primary heat exchanger. The reversible refrigeration system transfers heat energy between first and second secondary fluids which will typically comprise outdoor air and inside air which is to be conditioned.
The reversible refrigeration system, which may comprise a heat pump, includes a reciprocating compressor having a piston driven by the combustion gases of the heat engine. In most cases, the major portion of the pulse combustion energy is used in the primary heat exchanger, and generally no more than about 20% of the total energy is used for mechanical work. Virtually all of the heat energy not used to provide mechanical work is transferred in the primary heat exchanger due to the highly efficient heat transfer characteristics of pulse combustion associated with the relatively high turbulence and cyclic flow reversal of the combustion gases.
The pulse combustion process and technology of interest here are widely used in heating applications, such as water heaters, with combustion of natural gas or other gaseous fuels, as well as liquid fuels. In pulse combustion burners of the Helmholtz type, a combustion chamber of a given size is connected to an exhaust or tailpipe of given length having a cross section somewhat less than that of the combustion chamber. The tailpipe may be connected to an enlarged volume decoupler prior to venting the gases. The burner is designed to operate in a resonant manner at or near its natural frequency, as primarily determined by the geometry of the combustion chamber, tailpipe, and decoupler, if used, in accordance with the Helmholtz equation. The operating pressure is characterized by a sinusoidal curve. An oscillating or pulsatile flow of combustion gases through the burner is maintained by explosive combustion cycles in the chamber. The thermal expansion of the combustion gases drives the gases from the chamber and through the downstream elements to provide a self-exhausting operation or burner system. The burner may be arranged to provide self-feeding of fuel and combustion air using, for example, aerodynamic valves to provide a self-sustaining combustion process.
The pulse combustion process is particularly suited for driving reciprocating or oscillating mechanical elements, since the combustion process is characterized by alternating periods or cycles of higher and lower pressures or positive and negative pressure. The inherent alternation of pressures is available for driving or powering movement in each direction. As indicated, the driven element may be driven in either a reciprocating back-and-forth motion or an oscillating, rotary motion. The term "reciprocating" and variations thereof are used herein to include both types of motion unless such is clearly contrary to the context of the discussion.
The natural operation of pulse combustion to power reciprocating motion in both directions is especially efficient since the mechanical element, such as a piston, is driven on both half-cycles of the operating pressure curve of the burner. This also enables simplification of the componentry, since return "bounce" cylinders or other mechanical elements are not required. Similarly, piston movement automatically begins with combustion and it is not necessary to use a starting device to initially bias the piston as in the described prior art free piston devices.
The present invention is particularly advantageous in fluid conditioning systems which may include both liquids and gases. The invention is especially efficient in providing compressor operation for a heat pump system wherein nearly all of the energy not used in the compressor operation is recovered by heat exchange with a primary fluid. For convenience hereinafter, the invention is described with reference to a combined hot water heater and heat pump application, the latter being described with reference to the transfer of heat between indoor and outdoor air.